The process of sintering, manifested by transition of a porous set of powder particles into a solid material, is related to transport of mass in the porous set of particles. One of technologies of material production is powder metallurgy, in which, generally, powder compaction employs: free sintering, hot pressing or hot isostatic pressing.
Sintering by conventional methods often leads to grain growth, and as a consequence, to loss of properties resulting from the grain growth in the consolidated material. It happens particularly in the case of consolidation of materials with sub- and nanocrystalline grain size. Particularly significant grain growth during sintering nanocrystalline materials is observed after reaching critical density amounting 90% of the solid material's value. In result, it is difficult to obtain simultaneously a material with grain size below 100 nm and density close to theoretical by conventional sintering methods.
In the last 10-20 years a significant development of electric field activated sintering methods has occurred. These methods allow to conduct the sintering process in a very short time, from a few to over a dozen of minutes, limiting this way grain growth in the consolidated material. In the literature they are referred to as: Electro Discharge Compaction (EDC). Generally, these methods fall into methods with activation by electric field. In these techniques, similarly like in conventional hot pressing (HIP), the sintering process is realized with uni-axial pressure. A significant disadvantage of hot pressing is high temperature, long process time and small efficiency of heating of the consolidated powder. Moreover, high temperature and long consolidation process time are disadvantageous for obtaining materials with nanocrystalline microstructure. Electric field activated methods also vary in method of thermal energy transfer to the sintered material.
In conventional sintering, thermal energy is delivered through radiation and heat conduction, causing heating of the sintered material from the top of the sinter to its core. This heating method results in small speed and efficiency of heating.
In electric field activated sintering methods, like EDC, thermal energy is discharged directly in the whole volume of the sintered material. This heating method results in high energy efficiency of these methods, because of small losses of energy into the environment. Although heating of the die by current pulses is not different from direct current heating in conventional methods, heating of powder is much more complex. This is caused by many possible paths of current flow through the consolidated powder. In these methods there occur many phenomena activating the sintering process. Spark discharges remove the layer of oxides and adsorbed gasses from the surface of particles and form new contacts and necks by arc discharges. Locally, due to Joule heat generation, contacts and necks are formed, improving further compaction in the sintering process.
In EDC type the source of the energy is a capacitor or a capacitor battery rated for voltage of few thousand to tens of thousands volts. This solution has been disclosed e.g. in U.S. Pat. Nos. 4,929,415; 5,084,088; in which, as shown, a capacitor battery having capacitance of 240 μF has been used and the voltage of operation was between 3 and 30 kV. According to the disclosure and state of the art in the field of EDC type sintering methods, application of high voltage of the order of a few thousand volts is critical especially in the initial phase of the sintering process and is related to phenomena of spark discharges between grains of the powder being sintered. By charging and discharging of electric energy high-temperature spark or plasma discharges appear between powder particles. Pulse plasma activates the surface of the sintered particles, removes the oxide layer. In electric field activated sintering removal of oxides and later inter-particle connection occur due to miscellaneous phenomena of resistive heating from thermal and electrical breakdown of the isolating film to arc discharges. The arising difference of potentials between two particles becomes high enough for spark generation and releasing ionization process. Plasma generated between particles serves for activation of their surface by oxide and other impurity removal.
Advantages of EDC method include:                low temperature of sintering process,        shorter sintering process time,        speed of heating unattainable for other sintering techniques,        high thermal efficiency, which is defined by heating method, electric current is directly applied to the sample and electrically conductive die,        possibility of sintering powder materials impossible to produce by classic methods.        
EDC process uses oscillatory discharging of the capacitor battery for generation of current surges with first half-wave amplitude on the order of tens of kA and total discharge time ca. 1 ms. Operation cycle includes charging the capacitor battery to a given voltage value (from a few to tens of kV), and then oscillatory pulse discharge in the load circuit. EDC process is used to consolidate powder materials (Orrú R, Licheri R, Locci A M, Cincotti A and Cao G 2009 Mater. Sci. Eng. R 63 127), where the energy source is a high value capacitor, with capacitance of hundreds of microfarads. EDC is based on high-voltage discharge (to 30 kV), high pulse current density delivered directly from the capacitor battery with external pressure to the material being sintered, due to which a rapid increase of temperature and very quick sintering process are obtained. In these methods the energy stored in the capacitor battery is delivered to the sintered powder placed in the die and subjected to a simultaneous process of pressing. The cycle of charging the capacitor battery and subsequent discharging repeats with frequency limited on one hand by the power of supply unit charging the battery, and on the other hand by parameters of the spark gap closing the discharging circuit of the capacitor or capacitor battery. So far, in processes using pulsed electric discharging the impulses are initiated by triggering system comprising a triggering module and an air gap switch closing the electric circuit. The triggering module causes electric arcing between the initiating electrode and the receiving electrode. The presence of arcing enables the proper discharge between the main electrodes, which, because of current intensity of tens of thousands amperes, leads to a quick wearing-out of operational surfaces of both electrodes. This process is particularly intensive on edges of the electrodes, because of very high current densities. This results in accelerated wearing-out of the electrodes, and thereby significant decrease of durability of discharging circuitry comprising spark gaps. Their maximum frequency of operation is also limited, caused by presence of ionized air after each discharge. A subsequent discharge can occur only after removal of the ionized air from the space between the electrodes, what, depending on the spark gap's construction and operating voltage, takes at least 0.3 s. This time significantly limits the frequency of operation of spark gaps.
U.S. Pat. No. 3,670,137 discloses a method of increasing the fatigue resistance of an iron containing metallic body. The method comprises the step of spark sintering a surface layer of tungsten carbide thereto by positioning a mass of tungsten-carbide powder along said body, positioning an electrode in continuous direct contact with said powder but with a spacing from said body and intermittently effecting an interrupted impulsive spark discharge between said electrode and said body and among the particles of said powder and between the body and the particles of said mass proximal to said body. Said discharge body is dimensioned to fuse said particles to one another and said body without melting of said mass. Voltages of 50 and 120 V has been applied.
The method is applied in a device that permits control of the density of the sintered body by varying the mechanical pressure applied thereto, and provides means to control the density of the sample by regulating the frequency of the discharge at least during the early stages. To this end the direct-current source of the device is connected in series with the output winding of a transformer. This transformer forms part of a varying-frequency oscillator and is saturable to control this oscillator. The oscillator consists of a pair of push-pull transistors whose emitters are energized by a battery in series with respective sections of the primary winding of the transformer. The base of each transistor is serially connected with the energizing windings of the transformer and returned to the emitter via a suitable biasing resistor. The transformer is also provided with a control winding in series with a rectifier and a variable resistor for determining the degree of saturation of the core and thus the frequency of oscillation. The control circuit is bridged across the electrode, and detects the direct-current-voltage drop thereacross. This circuit is poled so as to increase the frequency of the oscillation should the density of the sintered body fall below a predetermined value.
A problem restricting commercial use of EDC type sintering methods is the dependence of spark gap breakdown voltage on environmental conditions, especially air humidity. With the increase of humidity the breakdown voltage decreases, and this in consequence results in loss of technological process repeatability and reflects in the quality of the manufactured product. This problem can be solved by placing the installation in an air-conditioned room with automatic humidity level stabilization. This causes increased production cost and is troublesome in industrial conditions.
Spark gaps pose also a problem with reaching proper speed and precision of control of technological process parameters. This relates e.g. to the necessity of change from a large value current surge to low value with simultaneous increase of frequency of these surges. In circuits of oscillatory capacitor battery discharging such change is achieved by modification of its charging voltage. For a discharge to occur between spark gap's electrodes it is necessary to modify the distance between them, and this requires a break in the technological process.
Despite disclosing more than 20 years ago in document U.S. Pat. No. 5,084,088 a device shown in FIG. 1a and numerous advantages of the EDC method, so far consolidation methods using energy stored in a capacitor battery are not used on an industrial scale. The problem of low durability of devices has been partly solved by application of the device disclosed in international patent application WO 2010/070623. In this device there is used a capacitor battery charged by a power supply and discharged through the sample being heated. Quick wearing-out of the electrodes, usually made of tungsten, molybdenum or copper, makes it impossible to fully utilize the advantages of EDC methods. The solution proposed in document WO 2010/070623 was to use a voltage transformer—FIG. 1b, connected between the switch shorting the capacitor battery, and the sintered sample. The use of voltage transformer provides a longer operation without failure, but simultaneously increases current pulse duration and decreases its maximal value—a current pulse from the state of the art is shown in FIG. 2; tf value is ca. 30 ms—thus making it impossible to utilize the advantages of the EDC process.